Erdem Alpay

Ala Nawaiseh

Why Shadows?Real world has shadowsMore control of the game’s feel

dramatic effects spooky effects

Without shadows the realism of a scene is lost

Spatial location of models can be ambiguous

Importance of Shadows

•Where is the light coming from?

•Where are the objects in relation to each other?

Importance of Shadows

•Where is the light coming from?

•Where are the objects in relation to each other?

•Shadows are being cast and the answers to these questions are easily found.

Common Real-Time Shadow Techniques

ShadowShadowvolumesvolumes

Light mapsLight maps

ProjectedProjectedplanarplanarshadowsshadows

HybridHybridapproachesapproaches

Easy To Implement and with good results when casting shadows onto one planeSquishes the Model onto a plane in just one matrix multiplicationVery Fast Can extend to cast onto multiple planes, but each plane will require another rendering of the model.

Planar Projected Shadows

Planar Projected Shadows

[x, y, z] is the difference between the center of the model and the light source. Ideally you want to use the distance from each vertex, However it is not as fast since the matrix will have to be computed on a per vertex basis instead of a per model basis.The vertices of the model are multiplied by the Planar Projection matrix.

Plane Eq: Ax + By + Cz + D = 0

Shadow Mapping Lance Williams published the idea in 1978

Completely image-space algorithm No knowledge of scene’s geometry is

required must deal with aliasing artifacts

Well known software rendering technique

Pixar’s RenderMan uses the algorithm Basic shadowing technique for Toy Story,

etc.

Two pass algorithm: First, render depth buffer from the light’s point-

of-view Essentially a 2D function indicating the

depth of the closest pixels to the light. The result is a “depth map” or “shadow

map”. Second, render scene from the eye’s point-

of-view For each rasterized fragment determine

fragment’s XYZ position relative to the light compare the depth value at light position XY

in the depth map to fragment’s light position Z

The Shadow Mapping

The Shadow Mapping Cont.

The Shadow Map Comparison Two values

A = Z value from depth map at fragment’s light XY position

B = Z value of fragment’s XYZ light position

If B is greater than A, then there must be something closer to the light than the fragment then the fragment is shadowed

If A and B are approximately equal, the fragment is lit

Shadow Mappingwith a Picture in 2D (1)

lightsource

depth map Z = A

depth map’s image plane

A < B shadowed fragment case

First step

Shadow Mappingwith a Picture in 2D (1)

lightsource

eyeposition

depth map Z = A

fragment’slight Z = B

depth map’s image plane

eye view image plane,a.k.a. the frame buffer

A < B shadowed fragment case

Second Step

Shadow Mappingwith a Picture in 2D (2)

lightsource

eyeposition

depth map Z = A

fragment’slight Z = B

depth map image plane

eye view image plane,a.k.a. the frame buffer

A B unshadowed fragment case

Shadow Mappingwith a Picture in 2D (3)

The depth map could be The depth map could be at a different resolution at a different resolution from the frame bufferfrom the frame buffer

This mismatch canThis mismatch canlead to artifactslead to artifacts

image precision mismatch!

An artifact is small error caused by the render process.

More realistic Example for mapping

1st Pass

More realistic Example for mapping

2nd Pass

Z-Buffer (Depth Buffer)Speeds up games by telling the graphics processor not to render objects that aren't actually in view, specifically objects or parts of objects which are covered up by other geometry. Ranges from 16bit to 32bit, and it can take up as much memory as an extra frame buffer. If depth test is enabled,

when a new color that arrives for a pixel is closer to the window than the one already in the depth buffer.

If it passes, it then replaces the value already in the depth buffer.

Stencil BufferIt's usually only 1bit or 8bit. The stencil allows applications to "tag" various regions of the frame buffer.The stencil buffer can tell the video card what it's allowed to redrawFor example, let's say you have an air simulation game where the view is from the cockpit. It would be a waste to render the cockpit interior every framethe cockpit interior is tagged with a stencil number > 1The stencil buffer tag will tell the video card to only redraw where it sees a 0 in the stencil buffer

What Is Shadow Volume

A volume of space formed by an occluderBounded by the edges of the occluder

First proposed by Frank Crow in 1977.

Notice that the “far”ends of the volumegoes to infinity

How does Shadow Volume works

Compute the shadow volume for all visible polygons from the light sourceAdd the shadow volume polygons to the scene database Tag them as shadow polygons Assign its associated light source

Approaches in shadow volumes

Initially set parity to 0 and shoot a ray from eye to P

Invert parity when the ray crosses the shadow volume boundary

Parity = 1 it is in shadow Parity = 0 it is not in Shadow

First approach : Parity Test When is a surface is inside shadow?

Approaches in shadow volumes

First approach : Parity Test Problems with Parity Test

How does Shadow Volumes Works

Consider a camera at any point outside of the shadow and a point being shaded. Shoot a ray from camera to the point. If the point is in the shadow volume it will have a positive value since the ray incremented the stencil buffer every time it entered the shadow volume (front facing), and decremented it when it left (back facing).

Second Approach : Counter•When is a surface is inside shadow?

How does Shadow Volumes Works

•If the camera is within the shadow volume the incrementing and decrementing must be reversed. •This however is extra work for a check to determine the location of the camera must be made.

Shadowing object

light source

eye position

0

0

1

1 2 2

3

Shadow Volumes

Step 1: Render scene Z-values

Shadow Volumes

Front face: +1

Step 2: Render shadow volume faces

Back face: -1

Shadow Volumes

Front face: ±0 (Depth test)Back face: ±0 (Depth test) = ±0

Shadow Volumes

Front face: +1Back face: ±0 (Depth test) = +1

±0

Shadow Volumes

Front face: +1Back face: -1 = ±0

±0+1

Shadow Volumes

±0+1±0

Step 3: Apply shadow mask to scene

ConclusionShadow Mapping: offers real-time shadowing effects Independent of scene complexity Does not mandate multi-pass as stenciled

shadow volumes do Ideal for shadows from spotlights

Shadow Volume: The addition of shadow volume polygons can

greatly increase your database size Using the stencil buffer approach, pixel fill

becomes a key speed factor More precise than shadow mapping Polygon count explodes with object complexity

of geometry and number of light sources

ReferencesShadow Mapping References Lance Williams, “Casting Curved Shadows

on Curved Surfaces,” SIGGRAPH 78 (paper) William Reeves, David Salesin, and Robert

Cook (Pixar), “Rendering antialiased shadows with depth maps,” SIGGRAPH 87 (paper)

http://developer.nvidia.com/docs/IO/1830/ATT/shadow_mapping.pdf

ReferencesShadow Volume References Lance Williams, “Casting Curved Shadows

on Curved Surfaces,” SIGGRAPH 78 (paper) William Reeves, David Salesin, and Robert

Cook (Pixar), “Rendering antialiased shadows with depth maps,” SIGGRAPH 87 (paper)

http://developer.nvidia.com/docs/IO/1830/ATT/shadow_mapping.pdf

Shadow Volume Reconstruction from Depth Maps Michael D. McCool University of Waterloo